package org.apache.lucene.search;

/**
 * Copyright 2005 The Apache Software Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;

/** A Scorer for queries with a required subscorer and an excluding (prohibited) subscorer.
 * <br>
 * This <code>Scorer</code> implements {@link Scorer#skipTo(int)},
 * and it uses the skipTo() on the given scorers.
 */
public class ReqExclScorer extends Scorer {
    private Scorer reqScorer, exclScorer;

    /** Construct a <code>ReqExclScorer</code>.
     * @param reqScorer The scorer that must match, except where
     * @param exclScorer indicates exclusion.
     */
    public ReqExclScorer(Scorer reqScorer, Scorer exclScorer) {
        super(null); // No similarity used.
        this.reqScorer = reqScorer;
        this.exclScorer = exclScorer;
    }

    private boolean firstTime = true;

    public boolean next() throws IOException {
        if (firstTime) {
            if (!exclScorer.next()) {
                exclScorer = null; // exhausted at start
            }
            firstTime = false;
        }
        if (reqScorer == null) {
            return false;
        }
        if (!reqScorer.next()) {
            reqScorer = null; // exhausted, nothing left
            return false;
        }
        if (exclScorer == null) {
            return true; // reqScorer.next() already returned true
        }
        return toNonExcluded();
    }

    /** Advance to non excluded doc.
     * <br>On entry:
     * <ul>
     * <li>reqScorer != null,
     * <li>exclScorer != null,
     * <li>reqScorer was advanced once via next() or skipTo()
     *      and reqScorer.doc() may still be excluded.
     * </ul>
     * Advances reqScorer a non excluded required doc, if any.
     * @return true iff there is a non excluded required doc.
     */
    private boolean toNonExcluded() throws IOException {
        int exclDoc = exclScorer.doc();
        do {
            int reqDoc = reqScorer.doc(); // may be excluded
            if (reqDoc < exclDoc) {
                return true; // reqScorer advanced to before exclScorer, ie. not excluded
            } else if (reqDoc > exclDoc) {
                if (!exclScorer.skipTo(reqDoc)) {
                    exclScorer = null; // exhausted, no more exclusions
                    return true;
                }
                exclDoc = exclScorer.doc();
                if (exclDoc > reqDoc) {
                    return true; // not excluded
                }
            }
        } while (reqScorer.next());
        reqScorer = null; // exhausted, nothing left
        return false;
    }

    public int doc() {
        return reqScorer.doc(); // reqScorer may be null when next() or skipTo() already return false
    }

    /** Returns the score of the current document matching the query.
     * Initially invalid, until {@link #next()} is called the first time.
     * @return The score of the required scorer.
     */
    public float score() throws IOException {
        return reqScorer.score(); // reqScorer may be null when next() or skipTo() already return false
    }

    /** Skips to the first match beyond the current whose document number is
     * greater than or equal to a given target.
     * <br>When this method is used the {@link #explain(int)} method should not be used.
     * @param target The target document number.
     * @return true iff there is such a match.
     */
    public boolean skipTo(int target) throws IOException {
        if (firstTime) {
            firstTime = false;
            if (!exclScorer.skipTo(target)) {
                exclScorer = null; // exhausted
            }
        }
        if (reqScorer == null) {
            return false;
        }
        if (exclScorer == null) {
            return reqScorer.skipTo(target);
        }
        if (!reqScorer.skipTo(target)) {
            reqScorer = null;
            return false;
        }
        return toNonExcluded();
    }

    public Explanation explain(int doc) throws IOException {
        Explanation res = new Explanation();
        if (exclScorer.skipTo(doc) && (exclScorer.doc() == doc)) {
            res.setDescription("excluded");
        } else {
            res.setDescription("not excluded");
            res.addDetail(reqScorer.explain(doc));
        }
        return res;
    }
}
